I. Field
The following description relates generally to wireless communication systems, and more particularly, to a method and apparatus for facilitating multi-user joint transmit-receive beamforming.
II. Background
In order to address the issue of increasing bandwidth requirements that are demanded for wireless communication systems, different schemes are being developed to allow multiple user terminals to communicate with a single access point by sharing the channel resources while achieving high data throughputs. Multiple In, Multiple Out (MIMO) technology represents one such approach that has recently emerged as a popular technique for next generation communication systems.
Wireless communication systems may become multiple-access systems capable of supporting communication with multiple user terminals by sharing the available system resources (e.g., bandwidth and transmit power). These wireless communication systems may simultaneously support communication for multiple wireless communication devices where each wireless communication device may communicate with one or more base stations via transmissions on an uplink and a downlink. The uplink, which is also referred to as a reverse link, refers to the communication link from the wireless communication devices to the base stations, and the downlink, which is also referred to as a forward link, refers to the communication link from the base stations to the wireless communication devices. Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, and orthogonal frequency division multiple access (OFDMA) systems.
From an information theory perspective, forward link transmission in wireless communication systems falls under the general scenario of broadcasting on a multi-user channel. It is known that for this scenario, serving multiple user terminals simultaneously with careful signal processing carried out at both the transmit and receive ends provides better performance than a TDMA approach, where only one user terminal is served per slot. For example, a good approach in single transmit antenna systems is super-position coding (SPC). In multiple transmit antenna systems such as MIMO systems, dirty-paper coding (DPC) has been identified as an optimal multi-user transmission strategy. DPC involves very high complexity as well as non-linear processing, where a modulo operation is required at both transmit and receive ends. Therefore, it is desirable to have linear beamforming schemes for multi-user terminal MIMO systems. However, current linear schemes are primarily Zero-Forcing (ZF)-based schemes, which may suffer from significant power loss.
Beamforming schemes for multi-user terminal MIMO systems typically involve each wireless communication device determining, and then feeding back to a base station, a transmit beamforming vector that it will use to communicate with the base station. The base station will then select all wireless communication devices to be served in the same slot by choosing all wireless communication devices with compatible transmit beamforming vectors. For example, the base station will choose wireless communication devices with transmit beamforming vectors having small spatial correlation so interference may be reduced.
An issue encountered with the above described approach is that when each wireless communication device decides on a respective transmit beamforming vector, the wireless communication device does not know what other wireless communication devices may be served with it in the same slot; therefore, a wireless communication device may select a beam that will maximize its own energy and neglect to take into account any interference that the selected beam will cause to other wireless communication devices. Consequently, interference mitigation is more or less taken into account only during the wireless communication device selection process by the base station, where the wireless communication devices with small spatial correlations are served together. In addition, the finite resolution of the transmit beamforming codebooks typically result in large residual interference, especially at a high signal to noise ratio (SNR) regime.
Consequently, it would be desirable to address one or more of the deficiencies described above.